/Engine/Plugins/Editor/UVEditor/Source/UVEditorTools/Public/Selection/UVToolSelectionAPI.h
These are functions that are used if a selection made in the applied mesh cannot be converted to an unwrap mesh selection due to unset UV elements (meaning that the selected portion does not exist in the unwrap mesh). Such selections are stored separately from all those that are able to be converted (which get stored with the usual unwrap selections). These "unset element" selections are ignored by many tools/actions but operated on by some others.
bool HaveUnsetElementAppliedMeshSelections() const